Switchgrass Cultivar, Yield, and Nutrient Removal Responses to Harvest Timing
Ashworth, A. J., F. L. Allen, J. L. Bacon, C. E. Sams, W. E. Hart, J. F. Grant, P. A. Moore, and D. H. Pote.  2017.  Agronomy Journal, Vol 109(6):1-8.

Abstract:
Finite nutrients, such as P and K are assumedly remobilized post-senescence in herbaceous feedstocks like switchgrass (Panicum virgatum L.) as a function of environmental signaling and genotype. Harvesting early during the maturation phase may result in yield reductions and higher nutrient removal in biomass depending on ecotype. Therefore, it is necessary to target harvest dates that optimize yield while minimizing nutrient removal per cultivar. Consequently, objectives were to compare yields from 2010–2011 on eight widely used and experimental upland and lowland genotype (whole plot) at two locations in Tennessee, to determine: (i) which harvest timing (split-plot) provides maximum yield; (ii) effects of harvest timing (mid- September, October, November, and late October) on overall total P and K removal; and, (iii) how results are affected by cultivar. Among all post-senescence harvests, yields peaked early November (13.2 Mg ha–1), which was greater than all other harvests (P < 0.05), with mid-October and late October not differing from one another. Because yields peaked in early November, P and K removal did not vary across harvest dates (despite both P and K concentrations declining mid-October). Lowland cultivars yielded 3.9 Mg ha–1 more biomass annually than upland entries, suggesting lowland cultivars are better suited to environments in the Southeast. Due to lower yields, P and K removals were lower for upland (Blackwell and C62), compared with lowland cultivars. Consequently, switchgrass can be harvested as early as mid-September without removing greater amounts of P and K, although variations within upland and lowland cultivars will likely occur.